Nowadays, with the increasing demand on high resolution of LCD, there is a trend towards narrowing the frame of a display panel.
Generally, the analog voltage applied to an internal pixel for revealing contents to be displayed is provided by a source driver IC with an external image data input interface, which is disposed in the frame area on the glass substrate. The source driver IC includes a plurality of output terminals which are connected to the pixel array by way of, for example, metal-thin-film wiring on the glass substrate.
Typically, the number of wires extending from a side of the pixel array conforms to the number of pixels in a row. However, the number of output terminals of the source driver IC arranged in parallel is less than the number of pixels in a row due to the size of the output terminals.
Accordingly, a demultiplexer operating by time division is used, as disclosed in Japanese Laid Open Patent Publication No. 2007-334109, to distribute a less number of terminals of the source driver IC to a greater number of wires at a side of the array.
FIG. 9 illustrates a conventional demultiplexer, and FIG. 10 exemplifies operations of the conventional demultiplexer, wherein FIG. 10A is a circuit diagram of the conventional demultiplexer, and FIG. 10B is a timing diagram of the conventional demultiplexer.
In the conventional demultiplexer 10 shown in FIG. 9 and FIG. 10, switching units M1˜M7 disposed on output lines Lout1˜Lout7 are switched in response to control signals CNT1˜CNT7 supplied to control lines Lcnt1˜Lcnt7. Accordingly, an input signal IN supplied to an input line Lin is selectively outputted via one of the output lines Lout1˜Lout7 as a corresponding one of the output signals Y1˜Y7. The switching units M1˜M7, for example, are implemented with n-channel field effect transistors.
As shown in FIG. 10B, during a D1 data period of the input signal IN supplied to the input line Lin, the control signal CNT1 is at a high level while the control signals CNT2˜CNT7 are at a low level; during a D2 data period of the input signal IN, the control signal CNT2 is at a high level while the control signals CNT1, CNT3˜CNT7 are at a low level; during a D3 data period of the input signal IN, the control signal CNT3 is at a high level while the control signals CNT1, CNT2, CNT4˜CNT7 are at a low level; during a D4 data period of the input signal IN, the control signal CNT4 is at a high level while the control signals CNT1˜CNT3, CNT5˜CNT7 are at a low level; during a D5 data period of the input signal IN, the control signal CNT5 is at a high level while the control signals CNT1˜CNT4, CNT6, CNT7 are at a low level; during a D6 data period of the input signal IN, the control signal CNT6 is at a high level while the control signals CNT1˜CNT5, CNT7 are at a low level; and during a D7 data period of the input signal IN, the control signal CNT7 is at a high level while the control signals CNT1˜CNT6 are at a low level. Therefore, the resulting output data with the output signal Y1 is D1; the resulting output data with the output signal Y2 is D2; the resulting output data with the output signal Y3 is D3; the resulting output data with the output signal Y4 is D4; the resulting output data with the output signal Y5 is D5; the resulting output data with the output signal Y6 is D6; and the resulting output data with the output signal Y7 is D7. In other words, the input signal IN is selectively outputted through one of the output lines Lout1˜Lout7.
For narrowing the frame of the LCD, it is necessary to limit the sizes of not only the source driver IC but also the layout size of the demultiplexer. The layout lines thus become as thin as a needle.
Furthermore, power saving is also an important issue for designing a display. For example, in the field of mobile phones, the recharging cycle of a battery is one of the issues that concerns an end user very much.
It is critical for a mobile phone to be power-efficient, but it is still necessary to reveal information such as current time, residual power of battery, etc. on the display even when the mobile phone is not working as being put through, navigating pages or checking emails. Therefore, the backlight of the display is turned off temporarily to save power and reflected external light is used for revealing the information. However, even if the mobile phone is operated under such a reflective mode, hundreds of microwatts of power is still consumed for standby recovery, and several to hundreds of watts of power is also consumed for telephonic communication.
In the above-mentioned reflective mode, a displaying method such as a conventional MIP (Memory in Pixel) technology is used to minimize power consumption, wherein the analog source driver IC is suspended for saving power while utilizing a memory circuit in a pixel to hold the displayed frame.
According to the MIP technology, data are stored in a one-bit (two-value) memory of each sub-pixel. By way of selectively combining one of two voltage levels and one of three elementary colors in each pixel, eight colors (8=2^3) can be realized. However, a typical source driver IC generally reveals each pixel with combinations selected from 64 voltage levels and 3 elementary colors, which results in about 262K colors (262,144=64^3). It is apparent that many colors are sacrificed in the MIP technology.
For solving such a problem, a multi-bit MIP technology is preferred. For example, if the memory in each sub-pixel is of six bits, the performance will be comparable to that of the typical source driver IC, i.e. 262K colors (262,144=(2^6)^3) for each pixel. Since a 6-bit memory is used in each sub-pixel but only one source line at a side of the array is provided for writing data to each sub-pixel, time division is required for distributing data to the memories of the sub-pixels with the aid of a demultiplexer. Due to the demand on high resolution, the size of each sub-pixel is limited to at most 100 microns. In other words, the demultiplexer has to be miniaturized to a certain extent.